Trimethylhydroquinone diester and its hydrolysed product, trimethylhydroquinone, are advantageously used as an intermediate of pharmaceuticals. Specifically, they are used as a raw material for vitamin E, an anti-oxidant for resins, higher fatty acids, higher alcohols and oils, and a polymerisation inhibitor for polymerisable monomers, thus considered important in the industrial point of view.
Japanese Patent Application Laid-open No. 7632/1972 (JP-A-47-7632) teaches a method for producing trimethylhydroquinone diester by reacting 2,6,6-trimethylcyclohexe-2-en-1,4-dione (ketoisophorone, KIP) with an acylating agent in the presence of a protonic acid catalyst or a Lewis acid catalyst.
In this method, however, the protonic acid or the Lewis acid used as a catalyst has to be neutralised and separated after the reaction, which renders the process complicated. Besides, the yield of the produced trimethylhydroquinone diester decreases owing to hydrolysis which occurs during the neutralisation step. The method further requires a discarding step, because the catalyst, neutralised after the reaction, has to be thrown away after every reaction. Thus, it is impossible to recycle the catalyst effectively. The Lewis acid catalysts inclusive of BF.sub.3 OEt.sub.2 are expensive, and thus should not be disposed of for industrial advantages. As for reaction equipment, the reactor has to be made of a corrosion-resistant material, such as a glass-lining reactor and other specially made reactors, which are inevitably expensive.